1. Field of the Invention
The present invention relates to bump bonders whereby bumps are formed on electrodes of semiconductor chips when mounting semiconductor chips on a wiring board.
2. Description of the Related Art
The existing methods for mounting semiconductor chips on wiring boards include the method! as shown in FIG. 3 of electrically connecting electrodes 14 of a semiconductor chip 13 with lead frames 15 of a wiring board using Au wire 16 and the method as shown in FIG. 9 of forming Au bumps 7 beforehand on electrodes 5 of semiconductor chip 4 and electrically connecting semiconductor chip 4 formed with these Au bumps 7 with electrodes 12 of wiring substrate 11 by means of conductive adhesive 10.
The device that is used to effect electrical connection with Au wires 16 is called a "wire bonder". The device that is used to effect electrical connection with Au bumps 7 is called a "bump bonder".
FIG. 5 shows how bonding is performed by a bump bonder.
First of all, as shown in FIG. 5, a ball 3 is formed by melting, using a spark current, the tip of an Au wire 2 that is drawn through a capillary 1. This ball 3 is joined to electrode 5 of semiconductor chip 4 by applying ultrasound simultaneously with heat and pressure; after bonding has been achieved, capillary 1 is moved to form a loop 6 of Au wire 2 directly above ball 3 as shown in FIG. 6. Au wire 2 is then severed to leave this loop 6 behind.
In a semiconductor chip 4, with bumps 7 consisting of balls 3 and loops 6 formed in this way using a bump bonder on all the electrodes 5 of semiconductor chip 4, the projecting end faces of bumps 7 are simultaneously flattened as shown in FIG. 7 by pressing all the bumps 7 onto a moulding base 8 of defined surface flatness so that all the bump heights are made uniform.
Next, a paste-like coating of conductive adhesive 10 is transferred to the periphery of bumps 7 of semiconductor chip 4 as shown in FIG. 8. At this point, conductive adhesive 10 whose binder is epoxy resin is provided in uniform thickness on the upper surface of moulding base 9 of defined flatness. When loops 6 that have been adjusted to uniform height are pulled up after being collectively pressure-fixed, a paste-like coating of conductive adhesive 10 is stuck onto the upper face of all the bumps 7.
Then, as shown in FIG. 9, mounting is completed by joining bumps 7 of semiconductor chip 4 in positionally-aligned condition to a plurality of conductive films 12 formed in a pattern on wiring board 11 by means of conductive adhesive 10 and heat-hardening conductive adhesive 10 by heating.
In the case of a bump bonder, it is necessary to form a ball 3 beforehand at the tip of Au wire 2, so, in forming the ball beforehand at the tip of Au wire 2, in order to obtain satisfactory bumps 7, discard bonds are required whereby any unsatisfactory balls 3 may be removed until satisfactory balls 3 can be produced.
In the discard-bond step, as shown in FIG. 4, unwanted wire or balls are bent at the capillary tip; bonding is performed to semiconductor chip 4 for discard bonding and thereafter a ball 3 is formed by spark application from battery 11 to wire 2 which is drawn out to a prescribed length.
In the case of a wire bonder, the operation of electrically connecting electrodes 14 of semiconductor chip 13 with lead frame 15 of the wiring board by Au wire 16 by discard-bonding in free areas 15a of lead frame 15 shown by virtual lines in FIG. 3 can be continued, but, in the case of bump bonding, the present situation is that, in the case of a semiconductor chip 4 of small surface area, it is not possible to secure enough area for the discard bonds that are to be removed.
An object of the present invention is to provide a bump bonder whereby satisfactory bumps 7 can be formed by discard-bonding of unwanted wires and/or unsatisfactory balls 3 by suppressing to a very low level the lowering of tact efficiency of formation of bumps 7.